The use of low powered portable devices is becoming increasingly common. For example, hand-held device such as cell phones, personal digital assistants (PDA), and the like are becoming increasingly common. Many microprocessor devices have been designed with one or more low power states in order to accommodate power conservation. By conserving power, battery life can be extended, thereby allowing a longer operating time for hand-held devices.
One method of implementing low power states in modem micro-controllers and microprocessors is to provide low power instructions. By executing a low power instruction, the device can initialize a shut down sequence, whereby the device prepares itself for shut down in such a manner that its subsequent return from a low power state can be accomplished. For example, if a low power microprocessor were to be shut down during a memory access cycle, it would be possible to disable the processor prior to an external device, such as a memory, returning the requested data. If upon startup the processor were expecting to see these completed cycle it could result in a lockup of the device since it will be anticipating a memory access which will not occur. However, by executing low power instructions, it is possible for state a machine associated with the processor to shut down in such a manner that it enters, and subsequently exits, low power modes in a known fashion.
While many microprocessor cores support the use of low power instructions to enter low power states in known conditions, the use of such instructions, and other methods is not universally available across all microprocessor cores. When it is desirable to use such microprocessor cores in low power devices, alternative methods for entering low power modes of operation need to be defined. Also, bus master devices such as DMA units and other system function blocks may not execute low power mode instructions directly, and placing such devices in a low power state in a similarly transparent fashion is also a requirement for low power systems. Therefore, the ability to enter a low power mode of operation is an efficient manner would be useful.